When sheathed cables are manufactured the sheath must after an extrusion of the sheath be vulcanized and cooled in one sequence. For this purpose a vulcanizing pipe and a subsequent cooling pipe are arranged after a plastic extruder through which the metal conductor that is to be sheathed is transported by means of a first capstan arrangement, arranged ahead of the extruder and a second capstan arrangement, arranged after the cooling pipe. At the vulcanizing stage the cable is supported in a horizontal extrusion process solely by the two capstan arrangements, or jointly by the first capstan arrangement, the cooling pipe and the second capstan arrangement. Therefore the cable has a catenary-like path at least in the vulcanizing pipe, whereby at least the vulcanizing pipe also has a catenary design.
To prevent the cable in the vulcanizing pipe to come into contact with the hot vulcanizing pipe it is necessary to keep the cable stretched so between the capstan arrangements that the cable runs through the vulcanizing pipe without coming into contact with it. A contact between the cable and the resistances would inevitably cause damage to the sheath of the cable and the cable would have to be discarded. This puts very great demands on the operation of the capstan arrangements, i.e. it requires that the capstan wheel carries the metal conductor without sliding between the conductor and the capstan wheel, that the stand is as robust as possible and rigidly anchored to the base, that the capstan wheel is firmly mounted in bearings on the stand to minimize the risk of vibrations and oscillations and that the driving engagement between the capstan wheel and the wheel drive for rotating the capstan wheel operates as smoothly and free from play as possible.
In previously known capstans of the above mentioned type the rotation of the capstan wheel is based on a gear ring-gear wheel arrangement between the capstan wheel and the wheel drive. Hereby the capstan wheel is provided with a gear ring with inner or outer teeth, which gear ring is attached to the capstan wheel co-axially with the rotation axis of the capstan wheel and the wheel drive is provided with a motor driven gear wheel with outer teeth, which is supported by the stand. The gear wheel is connected to a motor controlled by a tachometer generator or by a corresponding pulse transducer feedback through a suitable gear change, which motor is controlled via a control unit by a sensor arranged in the vulcanizing pipe for detecting the position of the sheathed cable in the pipe. The capstan arrangement positioned at the outlet end of the cooling pipe is usually operated at a constant speed, so that the position of the cable in the vulcanizing pipe can be adjusted by regulating the rotation of the capstan arrangement positioned ahead of the extruder.
However, practical experience has shown that such a gear driving design, despite a meticulous shaping of the teeth to achieve an engagement without play, causes vibrations in the capstan arrangement and thereby a harmful swinging of the cable in the vulcanizing pipe. This is apparently caused by play occurring all the same in the gear driving due to wear under great strain. It should be noted that the distance between the two capstan arrangements in such a cable vulcanizing line nowadays can be as long as 200 meters, whereby the hanging cable puts great strain on the capstan wheel and the teeth that have to support a tensional force that normally exceeds the weight of the cable. The difficulty of avoiding ovality in the ring-shaped gear ring also causes play variations in the engagement of the teeth and thereby disturbances in the uniform operation of the capstan wheel of the capstan arrangement.
In another known solution the capstan wheel has been totally replaced by a series of rollers aligned close to each other in a semicircular formation, mounted in bearings on a robust stand, over which rollers an inner endless rubber belt is running. An outer endless rubber belt is arranged on the outer side of the semicircular roller track, which belt is running over turning wheels and presses against the roller track in a semicircular formation. The metal conductor, around which a sheath is to be extruded, runs between the inner and outer belt from an unwinding device to an extruder. The speed by which the metal conductor moves into the extruder is regulated by a braking of the turning wheels over which the inner rubber belt is running. By this solution the disadvantage involved in the use of a play inducing gear driving can certainly be avoided, but the braking effect that a capstan wheel with a great peripheral contact angle provides is lost.
A substantial drawback with a capstan arrangement of that design is that the inner rubber belt is exposed to extremely high pressure due to pressure from the rollers and the outer rubber belt, which causes breakage in the inner rubber belt. An additional drawback is that the outer rubber belt easily slides to the side in the roller track and might break as the belt presses against a relatively thick metal conductor in the roller track.
In another design of a capstan arrangement a drawing mechanism in the form of a so called caterpillar is mounted in front of a capstan wheel rotatably mounted in bearings on a robust stand, through which caterpillar the metal conductor is fed. With such a design the braking effect and belt conditions can, however, be improved but space is lost due to the separate caterpillar arrangement, which in itself has teeth play.
In yet another design of a capstan arrangement having a rotatable capstan wheel mounted in bearings on a robust stand the rotation of the capstan wheel is effected via a worm gear mechanism provided with a motor connected to the central axis of the capstan wheel. The gear mechanism is here subjected to considerable momentums and must be very heavily dimensioned and the great play of the gear mechanisms is unavoidable.